Past years focus was on the dynamics of IRSp53 and cortactin, two proteins that interact with major PSD scaffolds as well as the actin cytoskeleton. The distributions of these proteins at the postsynaptic compartment in cultured hippocampal neurons were studied by immuno-electron microscopy under basal and excitatory conditions. Under basal conditions, label for IRSp53 is concentrated at the PSD complex, with a median distance of 30 nm from the postsynaptic membrane. Upon depolarization, the density of IRSp53 label at the PSD complex increased by 36%. Application of NMDA mimicked this effect. The accumulation of IRSp53 label upon application of high K+ or NMDA was especially prominent at the deeper region of the PSD complex, 40-120 nm from the plasma membrane, called the PSD pallium. IRSp53 molecules accumulated at the PSD pallium under excitatory conditions are too far from the membrane to fulfill the generally recognized role of the protein as an effector of membrane-bound small GTPases. These results suggest an additional structural role for IRSp53 as an adaptor at the interface of the PSD complex and the actin cytoskeleton. Cortactin is not enriched in PSD fractions, in agreement with observations by immuno-electron microscopy that indicate its widespread distribution within neurons. Also, in contrast to IRSp53, cortactin does not appear to redistribute within the postsynaptic compartment under the applied excitatory stimuli. These observations, together with the known binding properties of cortactin, suggest that the protein may peg Shanks to the actin cytoskeleton under basal conditions. We are currently investigating a role of CaMKII in disrupting the binding of Shanks to cortactin, a possible mechanism for the translocation of Shanks to the PSD under excitatory conditons.